Fixed NPE when Server has no Image (JCLOUDS-558)

Fixes: https://issues.apache.org/jira/browse/JCLOUDS-558
Follow up to: https://github.com/jclouds/jclouds/pull/414
[minor] How about
```
Something serverImage = serverInZone.getServer().getImage();
if (serverImage == null) {
   return null;
}
Image image = findObjectOfTypeForServerOrNull(images.get(), "image", serverImage.getId(), serverInZone);
return (image != null) ? image.getOperatingSystem() : null;
```
If this is a backport, this would obviously be something for a separate commit.